<lightning-layout-item padding="around-small">
            <lightning-input type="number" formatter="currency" name="amount" value={Custom_Object__c.Amount__c} onchange={handleChange} required></lightning-input>
    </lightning-layout-item>
    <lightning-layout-item padding="around-small">
            <lightning-input name="size" value={Custom_Object__c.Size__c} onchange={handleChange} required></lightning-input>
    </lightning-layout-item>

if (event.target.name === 'amount') {
        this.Custom_Object__c.Amount__c = event.target.value;
    }
    if (event.target.name === 'size') {
        this.Custom_Object__c.Size__c = event.target.value;
    }